The present invention relates to magneto-optical recording media, such as magneto-optical disks, magneto-optical tapes, and magneto-optical cards, for use in a magneto-optical recording and reproducing apparatus, and a method of reproducing information on such a magneto-optical recording medium.
Magneto-optical recording media have been in practical use as rewritable magneto-optical recording media. Information is recorded on and reproduced from the magneto-optical recording medium with the use of a light beam which is emitted by a semiconductor laser and converged on the magneto-optical recording medium. A problem with such conventional magneto-optical recording media is that their reproduction characteristics deteriorate when recording bits serving as magnetic recording domains have too small diameter and intervals, in comparison with the diameter of the light beam.
The cause of such a problem is that the light beam converged on a target recording bit covers within its diameter not only the target recording bit, but also a neighboring recording bit, and that each recording bit therefore cannot be reproduced independently.
Japanese Laid-Open Patent Application No. 150418/1994 (Tokukaihei 6-150418) discloses a magneto-optical recording medium capable of solving the above-mentioned problem. The magneto-optical recording medium is constructed to have a non-magnetic intermediate layer between a recording layer and a readout layer. The readout layer is in an in-plane magnetization state at room temperature, and changes into a perpendicular magnetization state with a rise in temperature. In this structure, magnetostatic coupling is achieved between the recording layer and readout layer through the non-magnetic intermediate layer therebetween. In a portion of the readout layer which is in the perpendicular magnetization state, the magnetization of the recording layer is copied. On the other hand, a portion of the readout layer, which exhibits the in-plane magnetization state, masks the magnetization of the recording layer. This structure does not allow reproduction of information of a recording bit in a portion of the recording layer, which is adjacent to the portion of the readout layer exhibiting the in-plane magnetization state. It is therefore possible to reproduce each recording bit independently even if the converged light beam covers the target recording bit to be reproduced and a neighboring recording bit within its diameter.
However, it has been confirmed that the magneto-optical recording medium disclosed in the above-mentioned document has a problem in reproducing information which is recorded with a smaller recording bit diameter at smaller recording bit intervals. Namely, sufficient masking cannot be achieved by the in-plane magnetization, and sufficient reproduced signals are not obtainable.
An object of the present invention is to provide a magneto-optical recording medium capable of offering reproduced signals of information recorded with a so small recording bit diameter at so small recording bit intervals as not to be reproducible from a conventional super-resolution magneto-optical recording medium, and provide a reproduction method.
In order to accomplish the object, a magneto-optical recording medium of the present invention has:
a recording layer made of a perpendicular magnetization film; and
a readout layer made of a magnetic film that is in an in-plane magnetization state at room temperature and changes into a perpendicular magnetization state from the critical temperature to the Curie temperature thereof, wherein a portion of the readout layer, which is in the perpendicular magnetization state, is magnetically coupled with the recording layer and copies the magnetization of the recording layer, and a portion in the in-plane magnetization state and a portion having a temperature not lower than the Curie temperature do not copy the magnetization of the recording layer.
In this magneto-optical recording medium, the critical temperature is a phase transition temperature at which the readout layer changes from the in-plane magnetization state to the perpendicular magnetization state. When the magneto-optical recording medium is irradiated with a light beam, three different temperature regions are formed in the readout layer. These temperature regions include a first-temperature region which is not exposed to the light beam and does not show a rise in temperature, a second-temperature region whose temperature is not lower than the critical temperature but is not higher than the Curie temperature of the readout layer, and a third-temperature region whose temperature is not lower than the Curie temperature.
In the first-temperature region, since the readout layer is in the in-plane magnetization state, the magnetization of the recording layer which is in the perpendicular magnetization state is not copied. In addition, in the third-temperature region, since the readout layer has a temperature not lower than the Curie temperature, the magnetization of the recording layer is not copied. Therefore, among the three different temperature regions of the readout layer, only the second-temperature region can copy the magnetization of the recording layer. It is thus possible to extremely narrow a region which copies the magnetization of the recording layer and is covered within the spot of the light beam. Consequently, even when the recording bit diameter and the recording bit intervals on the recording layer are very small, a target recording bit can be reproduced separately from a recording bit adjacent to the target recording bit, thereby achieving magnetic super-resolution reproduction of higher resolution.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.